1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus. More particularly, the present invention relates to an image forming apparatus including an image carrier, image forming devices for forming a toner image on the image carrier and an image transferring device for transferring the toner image to a sheet or recording medium, wherein the image carrier and one or more of the image forming devices are constructed into an image forming unit removably mounted to the body of the image forming apparatus.
2. Description of the Background Art
A color image forming apparatus of the type using an intermediate image transfer system belongs to a family of conventional image forming apparatuses. This type of image forming apparatus includes a plurality of photoconductive drums or image carriers and a single intermediate image transfer drum facing the drums. Toner images of different colors are sequentially transferred from the drums to the intermediate image transfer belt one above the other, completing a composite color image. The composite color image is then transferred to a sheet or recording medium.
It is likely that the toner images of different colors are shifted in position from each other on the intermediate image transfer belt, resulting in color shift. Color shift is a serious problem when it comes to a color image forming apparatus. To obviate color shift, it has been customary to accurately mount and affix the intermediate image transfer belt and drums to the apparatus body by use of a jig.
The intermediate image transfer belt whose life is usually shorter than the life of the apparatus body must be replaced when the degradation of performance or damage occurs before the life of the apparatus body ends. A specific conventional method of replacing the intermediate image transfer belt will be described hereinafter.
The intermediate image transfer belt is included in a belt unit, which forms part of an image forming unit, together with rollers over which the belt is passed and a frame surrounding the belt and rollers. When the belt unit is disposed in the apparatus body, axially opposite ends of each roller is accurately positioned. The ends of the rollers may be positioned by a specific configuration to be described hereinafter.
In the specific configuration, parallel pins are studded on opposite ends of each roller in such a manner as to extend in the axial direction of the roller. Guide holes are formed in the rear wall of the apparatus body while an opening is formed in the front wall and sized to allow the belt unit to be mounted or dismounted therethrough. A faceplate, which selectively opens or closes the above opening, is also formed with guide holes. Two arms extend from the bottom of the faceplate and are hinged to the apparatus body via a shaft. The faceplate is therefore angularly movable between a horizontal position parallel to a floor and a vertical position perpendicular to the horizontal position. The faceplate covers the opening in the vertical position or uncovers it in the horizontal position. When the belt unit is mounted to the apparatus body, the pins of the rollers each enter one of the guide holes to thereby accurately position the path of the intermediate image transfer belt inside the apparatus body.
For example, assume that developing devices and photoconductive drums are positioned above the intermediate image transfer belt. Then, to dismount the belt unit from the apparatus body, the faceplate is lowered to the horizontal position to uncover the opening with the guide holes of the faceplate being released from the front pins of the rollers. Subsequently, the belt unit is pulled out toward the front of the apparatus body. At this instant, the rear pins of the rollers are released from the guide holes of the rear wall as soon as the belt unit is slightly pulled out. As a result, the belt unit is made free. The belt unit is then fully pulled out of the apparatus body. To mount the belt unit, the belt unit is inserted into the apparatus body via the opening, which is uncovered then, until the rear pins of the rollers mate with the guide holes of the rear wall. Subsequently, the faceplate is raised toward the vertical position to cover the opening with the guide holes of the faceplate mating with the front pins. The belt unit is fully positioned when the faceplate reaches the vertical position.
The user of the apparatus must bear extra expenses every time the user calls a service person for, e.g., the replacement of the intermediate image transfer belt. It is therefore preferable to construct replaceable members into a single unit that can be replaced by the user.
The problem with the conventional belt unit described above is that the intermediate image transfer belt and drums are apt to contact and rub each other in the event of mounting or dismounting of the belt unit. More specifically, the pins of the rollers all are released from the guide holes as soon as the belt unit is slightly pulled toward the front of the apparatus body, as stated earlier. Therefore, if the belt unit shakes when being pulled out, then it is likely to move upward and contact and rub the drums and other members adjoining the intermediate image transfer belt. Such undesirable contact is apt to occur even when the belt unit is being mounted to the apparatus body. As a result, even a member that does not need replacement is scratched or otherwise damaged and must be wastefully replaced. This is particularly true when the user replaces the belt unit.
In light of the above, a space between the drums and the intermediate image transfer belt may be provided with a margin great enough to prevent the drums and intermediate image transfer belt from contacting each other even when the belt unit shakes during mounting or dismounting. This scheme, however, brings about another problem that the size of the apparatus increases in the direction of height.
The problems stated above arise not only with the intermediate image transfer belt but also with, e.g., a sheet conveying belt included in a belt unit mounted on a color image forming apparatus that uses a direct image transfer system.
Not only the belt unit and drums but also some other members must be prevented from contacting each other without a distance therebetween being increased. For example, when a developing device, facing any one of the drums, is removably mounted on the apparatus body, the developing device should preferably be prevented from contacting the drum. Also, when a cleaning unit, including the drum and a drum cleaner, or a process cartridge, including the drum and developing device, is removably mounted on the apparatus body, the cleaning unit or the process cartridge should preferably be prevented from contacting the drum. This is also true with an image forming unit or similar member removably mounted on the apparatus body and apt to shake and contact members therearound during mounting or dismounting.
To prevent the removable member from contacting members therearound, Japanese Patent Laid-Open Publication No. 9-325622, for example, proposes an image transferring unit angularly movable about a drive shaft between a contact position where it contacts photoconductive drums and a retracted position where the former is released from the latter. This document teaches that by moving the image transferring unit to the retracted position before the replacement of a drum unit, it is possible to prevent the drums and an image transfer belt from contacting each other. This proposal, however, also needs a margin in the distance between the image transferring unit and the drums when shaking to occur during mounting or dismounting is taken into account.
Japanese Patent Laid-Open Publication No. 2000-235309 discloses a belt unit, including an intermediate image transfer belt, removably mounted on an apparatus body and provided with a positioning block. When the belt unit is to be mounted to the apparatus body, the positioning block of the belt unit is positioned on a guide rail, and then the belt unit is inserted into the apparatus body. Subsequently, to set the belt unit in an operable position, a lift cam lifts the positioning block until the block abuts against a positioning shaft mounted on the apparatus body. As a result, the top of the belt and drums are positioned relative to each other. This allows the user of the apparatus to easily replace the intermediate image transfer belt while accurately positioning the drums and belt relative to each other. However, the positioning block is not lifted by the cam during mounting or dismounting of the belt unit, so that a space exists above at least the side of the belt unit where the positioning block is located. Consequently, the belt and drums are apt to contact each other when the user inadvertently lifts the belt unit during replacement of the belt.
Further, Japanese Patent Laid-Open Publication No. 2001-249508 teaches an image forming unit removably mounted on an apparatus body and made up of a photoconductive drum unit and an intermediate image transferring unit. Guide rails for a photoconductive drum and guide rails for an intermediate image transfer belt are arranged in the apparatus body. To mount or dismount the image forming unit to or from the apparatus body, drum bearings included in the image forming unit are positioned on the drum guide rails. At the same time, bearings mounted on opposite ends of a drive roller, which drives the intermediate image transfer belt, and lugs protruding from opposite side covers of the image forming unit are positioned on the belt guide rails. Subsequently, the image forming unit is slid into or out of the apparatus body. This configuration, however, is apt to bring about shaking because the image forming unit is slid with the bearings or the lugs contacting the guide rails, causing the image forming unit to contact members arranged in the apparatus body.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-110261, 11-84985, 11-295952 and 2000-194203.